Variable resistive element, method for producing the same, and nonvolatile semiconductor memory device including the variable resistive element

ABSTRACT

A variable resistive element configured to reduce a forming voltage while reducing a variation in forming voltage among elements, a method for producing it, and a highly integrated nonvolatile semiconductor memory device provided with the variable resistive element are provided. The variable resistive element includes a resistance change layer (first metal oxide film) and a control layer (second metal oxide film) having contact with a first electrode sandwiched between the first electrode and a second electrode. The control layer includes a metal oxide film having a low work function (4.5 eV or less) and capable of extracting oxygen from the resistance change layer. The first electrode includes a metal having a low work function similar to the above metal, and a material having oxide formation free energy higher than that of an element included in the control layer, to prevent oxygen from being thermally diffused from the control layer.

CROSS REFERENCE TO RELATED APPLICATION

This Nonprovisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2011-047236 filed in Japan on Mar. 4, 2011 theentire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nonvolatile variable resistiveelement including a first electrode, a second electrode, and a layerserving as a variable resistor formed of a metal oxide and sandwichedbetween the above electrodes, and a nonvolatile semiconductor memorydevice using the variable resistive element for storing information.

2. Description of the Related Art

Recently, as a high-speed operable next-generation nonvolatile randomaccess memory (NVRAM) to replace a flash memory, various devicestructures such as FeRAM (Ferroelectric RAM), MRAM (Magnetic RAM), andPRAM (Phase Change RAM) have been proposed, and they face intensedevelopment competition with a view to improving performance, increasingreliability, lowering cost, and ensuring consistency with processes.

With respect to these existing techniques, RRAM (Resistive Random AccessMemory) which is a nonvolatile resistive memory using a variableresistive element whose electric resistance is changed reversibly byapplying a voltage pulse has been proposed. FIG. 12 shows thisconfiguration.

As shown in FIG. 12, a conventional variable resistive element has astructure in which a lower electrode 103, a variable resistor 102, andan upper electrode 101 are laminated in this order, and it ischaracterized in that when a voltage pulse is applied between the upperelectrode 101 and the lower electrode 103, its resistance value can bereversibly changed. A new nonvolatile semiconductor memory device can berealized by reading a resistance value which is changed by thisreversible resistance changing action (hereinafter, referred to as the“switching action”).

The nonvolatile semiconductor memory device is composed by forming amemory cell array in which memory cells each including a variableresistive element are arranged in a shape of matrix in a row directionand a column direction, and by arranging periphery circuits to controlprogramming, erasing, and reading actions of data for each memory cellof the memory cell array. Thus, as for the memory cell, there are amemory cell in which one memory cell includes one selection transistor Tand one variable resistive element R (referred to as the “1T1R type”)and a memory cell in which one memory cell only includes one variableresistive element R (referred to as the “1R type”), depending on adifference in composed component. Among them, FIG. 13 shows aconfiguration example of the 1T1R type memory cell.

FIG. 13 is an equivalent circuit diagram showing one configurationexample of the memory cell array having the 1T1R type memory cells. Agate of the selection transistor T in each memory cell is connected to aword line (WL1 to WLn), and a source of the selection transistor T ineach memory cell is connected to a source line (SL1 to SLn) (n is anatural number). In addition, one electrode of the variable resistiveelement R in each memory cell is connected to a drain of the selectiontransistor T, and the other electrode of the variable resistive elementR is connected to the bit line (BL1 to BLm) (m is a natural number). Inaddition, the word lines WL1 to WLn are connected to a word line decoder106, and the source lines SL1 to SLn are connected to a source linedecoder 107, and the bit lines BL1 to BLm are connected to a bit linedecoder 105. Thus, in response to an address input (not shown), thespecific bit line, word line, and source line are selected forprogramming, erasing, and reading actions for the specific memory cellin a memory cell array 104.

Thus, according to the configuration in which the selection transistor Tand the variable resistive element R are arranged in series, thetransistor of the memory cell selected by a potential change of the wordline is turned on, and programming or erasing can be selectivelyperformed only for the variable resistive element R of the memory cellselected by a potential change of the bit line.

FIG. 14 is an equivalent circuit diagram showing one configurationexample of the 1R type memory cell. Each memory cell includes thevariable resistive element R only, and one electrode of the variableresistive element R is connected to the word line (WL1 to WLn), and theother electrode thereof is connected to the bit line (BL1 to BLm). Inaddition, the word lines WL1 to WLn are connected to the word linedecoder 106, and the bit lines BL1 to BLm are connected to the bit linedecoder 105. Thus, in response to the address input (not shown), thespecific bit line, and word line are selected for programming, erasing,and reading actions for the specific memory cell in a memory cell array108.

As for the above variable resistive element R, a method for reversiblychanging electric resistance by applying a voltage pulse to a perovskitematerial known for a supergiant magnetoresistance effect, as thevariable resistance material used in the variable resistor is disclosedin U.S. Patent Publication No. 6,204,139 (hereinafter, referred to asthe “well-known document 1”) by Shangquing Liu or Alex Ignatiev atHouston University in the United States, and in “Electric-pulse-inducedreversible Resistance change effect in magnetoresistive films”, AppliedPhysics Letter, Vol. 76, pp. 2749-2751, in 2000 by Liu, S. Q. et al. Bythis method, the several-digit resistance change appears in roomtemperature without applying an magnetic field even when the perovskitematerial known for the supergiant magnetoresistance effect is used. Inaddition, according to an element structure illustrated in thewell-known document 1, as the material of the variable resistor,praseodymium calcium manganese oxide Pr_(1-x)Ca_(x)MnO₃ (PCMO) filmwhich is a perovskite type oxide is used.

In addition, as another variable resistor material, an oxide of atransition metal element such as a titanium oxide (TiO₂) film, nickeloxide (NiO) film, zinc oxide (ZnO) film, or niobium oxide (Nb₂O₅) filmshows the reversible resistance change as shown in “Bistable Switchingin Electroformed Metal-Insulator-Metal Devices”, Phys. Stat. Sol. (a),vol. 108, pp. 11-65, in 1988 by H. Pagnia et al., and in “HighlyScalable Non-volatile Resistive Memory using Simple Binary Oxide Drivenby Asymmetric Unipolar Voltage Pulses”, IEDM 04, pp. 587-590, in 2004 byBaek, I. G. et al (hereinafter referred to as the “well-known document2”).

In addition, the above-described variable resistive element shows ann-type or p-type conductivity of a semiconductor because an impuritylevel is formed in a bandgap by an oxygen defect in the metal oxide. Inaddition, it is confirmed that the resistance change is a state changein the vicinity of an electrode interface.

As for the variable resistive element using the transition metal oxidefor the variable resistor, it is necessary to perform a soft breakdownprocess, referred to as a forming process to enable resistance switchingto be realized. A voltage (forming voltage) required for the softbreakdown process is higher than a programming voltage for recordinginformation. Meanwhile, since it is necessary to drive the variableresistive element with a fine transistor in realizing a highlyintegrated nonvolatile memory, the forming voltage has to be lowered.

Here, it is known that the forming voltage is roughly proportional to afilm thickness of the metal oxide used for the variable resistor, and amethod for lowering the forming voltage most easily is to thin the filmthickness of the metal oxide as disclosed in the well-known document 2.

However, when the film thickness of the metal oxide becomes thin, avariation in characteristics could be generated due to slightfluctuation of a film forming process or surface roughness of a basesubstrate.

SUMMARY OF THE INVENTION

In view of the above problems in the conventional technique, it is anobject of the present invention to provide a variable resistive elementhaving configuration such that low voltage can be realized while avariation of a forming voltage among elements is reduced, and a methodfor producing it.

It is another object of the present invention to provide a highlyintegrated nonvolatile semiconductor memory device that has the abovevariable resistive element and is easy to produce.

A variable resistive element according to the present invention toattain the above object is a variable resistive element having a firstmetal oxide film which is an oxide film of a first metal and sandwichedbetween a first electrode and a second electrode, in which

through a forming process, a resistance state between the first andsecond electrodes of the variable resistive element is changed from aninitial high resistance state before the forming process to a variableresistance state,

the resistance state in the variable resistance state is changed betweentwo or more different resistance states by applying an electric stressbetween the first electrode and the second electrode of the variableresistive element in the variable resistance state, and one resistancestate after the change is used for storing information,

a control layer containing oxygen is inserted between the firstelectrode and the first metal oxide film, and formed of a second metalcapable of extracting oxygen from the first metal oxide film to preventoxygen from being thermally diffused from the first metal oxide film tothe first electrode,

the second metal included in the control layer is different from thefirst metal, or same as the first metal,

when the second metal is same as the first metal, a concentrationdistribution is provided in such a manner that an oxygen concentrationof the control layer and the first metal oxide film becomes lower fromthe first metal oxide film toward the control layer across a boundarybetween the first metal oxide film and the control layer,

oxide formation free energy of at least one element included in thecontrol layer except for oxygen is lower than oxide formation freeenergy of an element included in the first electrode, and

both work functions of the second metal and the first electrode are 4.5eV or less.

As for the variable resistive element according to the present inventionhaving the above characteristics, it is preferable that a concentrationdistribution is provided in such a manner that an oxygen concentrationof the control layer becomes lower from the side of the first metaloxide film toward the side of the first electrode.

As for the variable resistive element according to the present inventionhaving the above characteristics, it is preferable that a concentrationdistribution is provided in such a manner that an oxygen concentrationof the first metal oxide film becomes lower from the side of the secondelectrode toward the side of the control layer.

As for the variable resistive element according the present inventionhaving the above characteristics, it is preferable that the first metaloxide film comprises an n-type metal oxide.

As for the variable resistive element according to the present inventionhaving the above characteristics, it is preferable that the first metaloxide film comprises an oxide of any one of elements of Hf, Zr, Ti, Ta,V, Nb, and W, or a strontium titanate.

As for the variable resistive element according to the present inventionhaving the above characteristics, it is preferable that the second metalcomprises any one of elements of Ti, V, Al, Hf, and Zr.

As for the variable resistive element according to the present inventionhaving the above characteristics, it is preferable that a work functionof the second electrode is 4.5 eV or more.

As for the variable resistive element according to the present inventionhaving the above characteristics, it is preferable that the controllayer is thinner than the first electrode.

As for the variable resistive element according to the present inventionhaving the above characteristics, it is preferable that a film thicknessof the control layer is 20 nm or less.

A method for producing the variable resistive element according to thepresent invention to attain the above object is a method for producingthe variable resistive element according to the present invention havingthe above characteristics, and it has the steps of;

depositing a second electrode material on a substrate, and forming thesecond electrode;

depositing a first metal oxide film material, a second metal material,and a first electrode material in this order;

forming the first metal oxide film and the first electrode by patterningthe first metal oxide film material, the second metal material, and thefirst electrode material with a common resist mask;

performing a heat treatment; and

applying a forming voltage between the first electrode and the secondelectrode to perform a forming process, whereby oxygen in the firstmetal oxide film being partially moved toward the second metal material,the second metal material being changed into the control layer, aresistance state of the variable resistive element being changed fromthe initial high resistance state to the variable resistance state.

A nonvolatile semiconductor memory device according to the presentinvention to attain the above object includes a memory cell array havingthe variable resistive elements according to the present inventionhaving the above characteristics, the variable resistive elements beingarranged in at least a column direction of a row direction and thecolumn direction.

As for the nonvolatile semiconductor memory device according to thepresent invention having the above characteristics, it is preferablethat the memory cell array is provided in such a manner that the firstelectrode extends in the column direction to connect the variableresistive elements adjacent in the column direction, and the controllayer extends in the column direction.

A dedicated study by the inventors of this application has found that inthe variable resistive element having the metal oxide film (variableresistor) sandwiched between the first electrode and the secondelectrode, the forming voltage can be reduced when the control layerformed of the metal film containing oxygen is provided between the firstelectrode and the metal oxide film, and the control layer extractsoxygen from the metal oxide film.

Thus, since the forming voltage can be reduced without depending only onthinning the metal oxide, it is not necessary to extremely thin the filmthickness of the metal oxide, and a margin is allowed for the control ofthe film forming process. In addition, the forming voltage can be stablylowered even for a heat history experienced in a semiconductor processby controlling a thickness of the control layer, and laminating thereonan electrode whose reactivity with oxygen is lower than that of thecontrol layer.

Therefore, according to the present invention, the forming voltage canbe lowered while a variation in forming voltage between the elements isreduced, and the variable resistive element can be driven at a lowvoltage, in actions including a forming process. As a result, thevariable resistive element can be easily driven with a fine transistorhaving a low withstand voltage, and the highly integrated nonvolatilesemiconductor memory device having the variable resistive element can beeasily realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional schematic view showing one example of astructure of a variable resistive element according to the presentinvention.

FIG. 2 is a table showing oxide formation free energy and a workfunction value of a metal.

FIG. 3 is a view showing a cumulative probability distribution of aforming voltage in a conventional variable resistive element.

FIG. 4 is a view showing a variation of a current amount flowing in thevariable resistive element with respect to a voltage applied to thevariable resistive element at the time of measurement of the formingvoltage.

FIG. 5 is a view showing a variation of a current amount flowing in thevariable resistive element with respect to a voltage applied to thevariable resistive element at the time of measurement of the formingvoltage.

FIG. 6 is a view showing an oxygen concentration distribution in thevicinity of a boundary between a first electrode and a resistance changelayer in a case where a heat treatment is performed and in a case whereit is not performed, in a conventional variable resistive element usingTa for the first electrode.

FIG. 7 is a view showing an oxygen concentration distribution in thevicinity of a boundary between the first electrode and the resistancechange layer in a case where a heat treatment is performed and in a casewhere it is not performed, in a conventional variable resistive elementusing Ti for the first electrode.

FIG. 8 is a view showing a cumulative probability distribution of theforming voltage in the variable resistive element according to thepresent invention.

FIG. 9 is a view showing writing endurance characteristics of thevariable resistive element according to the present invention.

FIG. 10 is a circuit block diagram showing a schematic configuration ofa nonvolatile semiconductor memory device according to the presentinvention.

FIG. 11 is a cross-sectional view showing a schematic structure of amemory cell array including the variable resistive element according tothe present invention.

FIG. 12 is a schematic view showing an element structure of aconventional variable resistive element.

FIG. 13 is an equivalent circuit diagram showing one configurationexample of a 1T1R type memory cell.

FIG. 14 is an equivalent circuit diagram showing one configurationexample of a 1R type memory cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT First Embodiment

FIG. 1 is a cross-sectional view schematically showing an elementstructure of a variable resistive element 2 used in a nonvolatilesemiconductor memory device according to one embodiment of the presentinvention (hereinafter, referred to as the “device 1 of the presentinvention” occasionally). In addition, in the following drawings, anessential part is emphasized for descriptive convenience and adimensional ratio of each part of the element does not always coincidewith an actual dimensional ratio.

According to this embodiment, a hafnium oxide (HfO_(X)) which serves asan insulator layer having a wide bandgap is used for a resistance changelayer (variable resistor). However, the present invention is not limitedto this configuration. The resistance change layer may include azirconium oxide (ZrO_(X)), titanium oxide (TiO_(X)), tantalum oxide(TaO_(X)), vanadium oxide (VO_(X)), niobium oxide (NbO_(X)), tungstenoxide (WO_(X)), or strontium titanate (SrTiO_(X)). In addition, theseare all an n-type metal oxide.

In addition, in the case where these transition metal oxides are usedfor the resistance change layer, initial resistance of the variableresistive element just after produced is very high, so that in order toput it into a state in which a high resistance state and a lowresistance state can be switched by an electric stress, it is necessaryto perform what we call a forming process in which a voltage pulse whichis large in voltage amplitude and long in pulse width compared to avoltage pulse used in a normal writing action is applied to the variableresistive element in the initial state just after produced, and acurrent path is formed to cause resistance switching before used. It isknown that the current path (referred to as a filament path) formed bythis forming process determines electric characteristics of the elementthereafter.

The variable resistive element 2 is formed such that a second electrode12, a first metal oxide film 13 which serves as a resistance changelayer and an oxide film of a first metal, a control layer 14 which is ametal film containing oxygen, and a first electrode 15 are deposited andpatterned in this order on an insulation film 11 formed on a substrate10. Here, the variable resistive element 2 is configured such that aShottky interface is formed at an interface between the second electrode12 and the resistance change layer (first metal oxide film) 13, and anelectron state in the vicinity of the interface is reversibly changed byapplying the electric stress and as a result, the resistance is changed.

The control layer 14 has an ability to extract oxygen from theresistance change layer 13, and contains oxygen in a second metal whosework function is smaller than that of the second electrode 12 so thatthe resistance switching can be stably caused at the interface betweenthe second electrode 12 and the resistance change layer 13. In addition,the control layer 14 may be an oxide film of the second metal (secondmetal oxide film), or a film of the second metal whose oxygenconcentration is not as high as the oxide, but which contains a lot ofoxygen as an impurity. The resistance change layer 13 and the controllayer 14 have concentration distributions in which an oxygenconcentration decreases from a side of the second electrode 12 to a sideof the control layer 14 in the resistance change layer 13, and from aside of the resistance change layer 13 to a side of the first electrode15 in the control layer 14. According to this embodiment, the controllayer 14 is configured such that the second metal film having theability to extract oxygen is in contact with the resistance change layer13, and oxygen in the resistance change layer (first metal oxide film)13 is partially moved toward the metal film by application of heat and aforming voltage. More specifically, materials of the control layer 14and the second electrode 12 are selected so that the work function ofthe second metal is 4.5 eV or less, and the work function of the secondelectrode 12 is 4.5 eV or more.

Here, the material of the second metal which can be used for the controllayer 14 and likely to extract oxygen includes Ti (4.3 eV), V (4.3 eV),Al (4.2 eV), Hf (3.9 eV), or Zr (4.1 eV). In addition, the material ofthe second electrode includes a titanium nitride (TiN: 4.7 eV) ortitanium oxynitride, or as a material which is relatively high in workfunction and used often in an LSI production process, tantalum nitride(TaNx: 4.05 to 5.4 eV depending on a stoichiometric composition x ofnitride), tantalum oxynitride, titanium aluminum nitride, W (4.5 eV), orNi (5.2 eV). In addition, a work function value of each metal is shownin parentheses.

Furthermore, a film thickness of the control layer 14 is set to apredetermined film thickness or less (preferably, equal to or less thana film thickness of the first electrode 15 and in the case of thisembodiment where TiOx is used for the control layer, more preferablyequal to or less than 20 nm) so that oxygen is not excessively extractedfrom the resistance change layer 13. Here, when the control layer 14 isthin, the electron state at the interface between the resistance changelayer 13 and the control layer 14 is affected by the first electrode 15.In this case also, a work function of the first electrode 15 is set tobe the same level as the work function of the second metal so as torealize stable resistance switching at the interface between the secondelectrode 12 and the resistance change layer 13. That is, a material ofthe first electrode is selected so that the work function of the firstelectrode 15 is also 4.5 eV or less. In addition, the first electrode 15is formed of a material having an oxygen extracting ability smaller thanthat of the control layer 14 so that the control layer 14 dominates theoxygen extraction.

Therefore, the material of the first electrode 15 is preferably selectedfrom materials having oxide formation free energy higher than oxideformation free energy of at least one element included in the controllayer 14 except for oxygen. In addition, the oxide formation free energyof the first electrode 15 is preferably higher than the oxide generationfree energy of at least one element of the control layer 14 except foroxygen by 100 kJ/mol or more per 1 mole of oxygen molecules. In thismanner, oxygen is prevented from being thermally diffused from thecontrol layer 14 to the first electrode 15.

FIG. 2 shows a value of the oxide formation free energy [kJ/mol] per 1mole of oxygen molecules, and a work function value at 427° C. (700K) ofan oxide of each of Ta, Ti, V, Al, W, Nb, Hf, and Zr, as the materialwhich can be used for the first electrode 15. As shown in FIG. 2, theoxide formation energy decreases in the order of Hf, Al, Zr, Ti, Ta, Nb,V, and W. For example, when TiOx is used for the control layer 14, amaterial such as Ta, Nb, V, or W can be used for the first electrodematerial.

Hereinafter, a method for producing the variable resistive element 2will be described. First, a silicon oxide film having a thickness of 200nm is formed on a single crystal silicon substrate 10 as the insulationfilm 11 by a thermal oxidation method. Then, as the material of thesecond electrode 12, a titanium nitride film having a thickness of 100nm is formed on the silicon oxide film 11 by a sputtering method.

Then, as the material of the resistance change layer (first metal oxidefilm) 13, a hafnium oxide film having a thickness of 3 to 5 nm (5 nm inthis case) is formed on the titanium nitride film 12 by sputtering orALD (Atomic Layer Deposition), and then as the second metal materialwhich becomes the control layer 14, a titanium film having a thicknessof 3 to 20 nm is formed by sputtering.

Then, as the material of the first electrode 15, a tantalum thin filmhaving a thickness of 150 nm is formed on the control layer 14 by asputtering method. Finally, a pattern is formed in a photoresistprocess, and the element region of 0.4 μm×0.4 μm is formed as shown inFIG. 1 by dry etching. Thus, the variable resistive element 2 isproduced. After that, a heat treatment is performed, and when needed, aninterlayer insulation film is formed and wiring is performed.

Then, the variable resistive element 2 is put into a variable resistancestate in which the resistance can be changed, by applying the formingvoltage between the first electrode and the second electrode.

At this time, oxygen in the resistance change layer 13 is partiallymoved to the titanium film at the time of the heat treatment and at thetime of application of the forming voltage, and as a result, thetitanium film is oxidized and becomes TiO_(X), whereby the control layer14 is formed.

Hereinafter, a description will be made of the fact that the aboveconfiguration is effective to solve the problem. FIG. 3 shows acumulative probability distribution of voltages after the formingprocess has been completed for 64 elements in a wafer surface, withrespect to conventional variable resistive elements 3 a to 3 c producedby the method for producing the variable resistive element 2 except forthe formation of the control layer 14. In addition, FIG. 3 shows aresult provided after the elements 3 a to 3 b have been subjected to aheat treatment at 400° C. Furthermore, the experiment is carried outsuch that with a semiconductor parameter analyzer (4156C produced byAgilent Technologies), a voltage when a current amount exceeds a certainvalue has been measured while increasing the applied voltage from 0V to5V by a step of 10 mV.

By making a comparison between the element 3 a in which a HfOx having afilm thickness of 5 nm is used for the resistance change layer 13, andTa having a thickness of 100 nm is used for the first electrode 15, andthe element 3 b in which HfOx having a film thickness of 5 nm is alsoused for the resistance change layer, and Ti having a thickness of 100nm is used as the first electrode instead of Ta, it is found that theforming voltage can be lowered in the case where the Ti electrode isused compared to the case where the Ta electrode is used. This isbecause Ti is likely to extract oxygen from the metal oxide compared toTa.

Meanwhile, by making a comparison between the element 3 a, and element 3c in which HfOx having a film thickness of 2 nm is used for theresistance change layer, and Ta having a thickness of 100 nm is used forthe first electrode, it is found that when HfOx is thinned, the formingvoltage can be reduced, but a defect in which a withstand voltage isextremely low is generated.

From the above, it is found that there is a limit on thinning the filmthickness in order to lower the forming voltage, and to use theelectrode which can easily extract oxygen is effective in lowering theforming voltage without generating an element having a defectivewithstand voltage.

FIGS. 4 and 5 are views each showing a variation of a current amountflowing in the variable resistive element with respect to the voltageapplied to the variable resistive element while the forming voltage ismeasured, and shows how the forming voltage is changed due to adifference in heat treatment temperature. In addition, FIG. 4 shows thecase of the element 3 a in which Ta is used for the first electrode 15in the element structure of the variable resistive element, and FIG. 5shows the case of the element 3 b in which Ti is used for the firstelectrode 15. In the drawing, a voltage when a current abruptlyincreases is a measurement value of the forming voltage.

From FIGS. 4 and 5, it is found that the forming voltage tends todecrease due to a heat history in the variable resistive element, butthe forming voltage largely decreases in the element 3 b using the Tielectrode (FIG. 5) compared to the element 3 a using the Ta electrode(FIG. 4) even when the heat history is the same. In addition, FIG. 5shows the result in the case where the heat treatment is performed up to350° C., but when the element 3 b is subjected to a heat treatment at420° C., some variable resistive elements short out.

That is, the above experiment result shows that to use the electrodesuch as Ti which is likely to extract oxygen, is effective to reduce theforming voltage, but when the heat history of the general semiconductorprocess is imposed, the variable resistive element could short out inthe case of the Ti electrode.

FIGS. 6 and 7 show oxygen concentration distributions in the vicinity ofthe boundary between the first electrode and the resistance change layermeasured by SIMS (Secondary-Ion Mass Spectroscopy) in the case where theelement 3 a and the element 3 b have been subjected to the heattreatment and the case where they are not subjected thereto,respectively. As can be understood from FIGS. 6 and 7, it is found thatoxygen is largely diffused toward the electrode in both cases where theheat treatment is performed and not performed in the element 3 b usingthe Ti electrode (FIG. 7) compared to the case of the element 3 a usingthe Ta electrode (FIG. 6). The oxygen concentration tends to decreasefrom HfOx toward the inside of the electrode in both of the elements 3 aand 3 b, but in the case of the element 3 b using the Ti electrode (FIG.7), oxygen enters the electrode deeply, while in the case of the element3 a using the Ta electrode (FIG. 6), both entered amount and entereddepth are smaller than those of the element 3 b.

Meanwhile, FIG. 8 shows a cumulative probability distribution ofvoltages after the forming process has been completed for 64 elements,with respect to variable resistive elements 2 a to 2 e of the presentinvention formed such that TiOx is formed as the control layer 14 onHfOx having a film thickness of 5 nm as the resistance change layer,according to the method for producing the variable resistive element 2.In addition, these elements have been subjected to a heat treatment at420° C. (to 700K) as the heat history of the semiconductor process. Itis found that in the case where Ta is used for the first electrode, asthe TiOx of the control layer becomes thick, the forming voltage becomeslow and can be reduced to 2V or less. Thus, these elements do not shortout due to the heat treatment at 420° C. and show preferable resistanceswitching.

Meanwhile, in a case of an element 4 a in which titanium nitride is usedfor the first electrode 15 and formed on the control layer 14 of TiOxhaving a thickness of 3 nm, its withstand voltage tends to increase. Inaddition, it does not show preferable resistance switching. It isconsidered that an influence by titanium nitride having the high workfunction appears as an effective electrode because the control layer 14is thinned. That is, it shows that it is important to provide the firstelectrode 15 having the low work function on the control layer 14.

As described above, when the variable resistive element 2 is formed bylaminating the first electrode 15 having the function to prevent oxygenfrom being thermally diffused and having the low work function, on thethin control layer 14, the variable resistive element can prevent oxygenfrom being thermally diffused from the control layer 14 toward the firstelectrode 15, suppress the forming voltage, and be stable to the heathistory of the general semiconductor process.

FIG. 9 shows writing characteristics in a 1T-1R configuration in whichthe variable resistive element 2 b using TiOx having a thickness 10 nmas the control layer 14 is connected to a MOSFET in series. It has beenconfirmed that stable writing can be performed until the number of timesof writing is 10⁷ times.

Second Embodiment

FIG. 10 shows an example of the device 1 of the present invention havingthe above variable resistive element 2 (2 a to 2 e). FIG. 10 is acircuit block diagram showing a schematic configuration of the device 1of the present invention. The device 1 of the present invention includesa memory cell array 21, a control circuit 22, a voltage generationcircuit 23, a word line decoder 24, and a bit line decoder 25.

The memory cell array 21 is the memory cell array represented by theequivalent circuit diagram shown in FIG. 13 or 14, in which memory cellseach including the variable resistive element 2 (any one of 2 a to 2 e)are arranged in a shape of a matrix in row and column directions, thememory cells belonging to the same column are connected by the bit lineextending in the column direction, and the memory cells belonging to thesame row are connected by the word line extending in the row direction,and when a selected word line voltage or an unselected word line voltageis applied to the word line, and a selected bit line voltage or anunselected bit line voltage is applied to the bit line, one or morememory cells, as an action target which has been designated by anexternal address input can be selected in each action such asprogramming, erasing, reading, and the forming process.

In addition, the memory cell array 21 may be any one of a memory cellarray having a 1R structure which does not include a current limitelement in a unit memory cell (refer to FIG. 14), a memory cell arrayhaving a 1D1R structure which includes a diode as a current limitelement in a unit memory cell, and a memory cell array having a 1T1Rstructure which includes a transistor as a current limit element in aunit memory cell (refer to FIG. 13). According to the memory cell arrayhaving the 1D1R structure, one end of the diode is connected to oneelectrode of a variable resistive element in series, to compose thememory cell, and the other end of the diode or the other electrode ofthe variable resistive element is connected to the bit line or the wordline. According to the memory cell array having the 1T1R structure,either a source or a drain of the transistor is connected to oneelectrode of the variable resistive element in series to compose thememory cell, and one of the other of the source or the drain of thetransistor which is not connected to the variable resistive element, andthe other electrode of the nonvolatile variable resistive element whichis not connected to the transistor is connected to the bit lineextending in the column direction, and the other thereof is connected toa common source line to supply the ground voltage, and gate terminals ofthe transistor are connected to the word line extending in the rowdirection.

The control circuit 22 controls each memory action in the memory cellarray 21 of programming (set), erasing (reset), and reading actions, andcontrols the forming process. More specifically, the control circuit 22controls the word line decoder 24 and the bit line decoder 25 based onan address signal inputted from an address line, a data input inputtedfrom a data line, and a control input signal inputted from a controlsignal line, and controls each memory action of the memory cell and theforming process. In addition, although not shown in the example shown inFIG. 10, the control circuit 22 is provided with functions as a generaladdress buffer circuit, data input/output buffer circuit, and a controlinput buffer circuit.

The voltage generation circuit 23 generates the selected word linevoltage and the unselected word line voltage, supplies them to the wordline decoder 24, generates the selected bit line voltage and theunselected bit line voltage, and supplies them to the bit line decoder25 to select the memory cell of the action target, at the time of eachmemory action of programing (set), erasing (reset), and reading, and theforming process of the memory cell.

When the memory cell of the action target is inputted to the addressline and designated, the word line decoder 24 selects the word linecorresponding to the address signal inputted to this address line, andapplies the selected word line voltage and the unselected word linevoltage to the selected word line and the unselected word line,respectively, at the time of each memory action of programing (set),erasing (reset), and reading, and the forming process of the memorycell.

When the memory cell of the action target is inputted to the addressline and designated, the bit line decoder 25 selects the bit linecorresponding to the address signal inputted to this address line, andapplies the selected bit line voltage and the unselected bit linevoltage to the selected bit line and the unselected bit line,respectively, at the time of each memory action of programing (set),erasing (reset), and reading, and the forming process of the memorycell.

In addition, as for a detailed circuit configuration, a devicestructure, and a production method of each of the control circuit 22,the voltage generation circuit 23, the word line decoder 24, and the bitline decoder 25, since they can be realized with the well-known circuitconfigurations, and produced by the well-known semiconductor productiontechniques, their description is omitted.

FIG. 11 shows a cross-sectional structure view of one example of thememory cell array 21 including the variable resistive elements 2 of thepresent invention. A memory cell array 21 a shown in FIG. 11 is thememory cell array having the 1T1R structure in which the first electrode15 extends in the column direction (lateral direction in FIG. 11) as thebit line BL, and the resistance change layer 13 and the control layer 14also extend in the column direction. A contact plug connected to atransistor T formed in an lower layer through an island-shaped metalwiring 31 and a contact plug 32 serves as the second electrode 12 whichis in contact with the resistance change layer 13. Thus, the variableresistive element 2 which includes the second electrode 12, theresistance change layer 13, the control layer 14, and the firstelectrode 15 is formed in a contact part (element formation region)between the second electrode 12 and the resistance change layer 13.

In addition, since the resistance change layer 13 which is in contactwith the second electrode 12 extends in the column direction, it isphysically in contact with the second electrode 12 of the adjacentvariable resistive element 2, but as described above, since thetransition metal oxide included in the resistance change layer 13 is aninsulator when the film is formed, after the forming process has beenperformed by applying the voltage between the first electrode 12 and thesecond electrode 15 of the variable resistive element 2 which is in aninitial high resistance state just after the film formation, itsresistance is lowered, so that the memory action as the variableresistive element 2 can be performed. Therefore, the resistance changelayer 13 positioned other than the element formation region is still inthe high resistance state even after the forming process, so that thereis no problem with a leak current.

In addition, it has been described that in the memory cell array havingthe 1T1R structure, the source line is shared by the all memory cellsand the ground voltage is supplied thereto in the above embodiment, butthe source line may extend in the column direction and connect memorycells belonging to the same column or extend in the row direction andconnect the memory cells belonging to the same row. Furthermore, when asource line decoder 26 (not shown) which applies a selected source linevoltage and an unselected source line voltage supplied from the voltagegeneration circuit 23 to source lines is further provided, the memorycell of the action target can be selected by designating the memory cellwith respect to each row or column at the time of memory actions ofprogramming (set), erasing (reset), and reading, and the formingprocess. When the memory cell of the action target is inputted to theaddress line and designated, the source line decoder 26 selects thesource line corresponding to the address signal inputted to this addressline, and applies the selected source line voltage and the unselectedsource line voltage to the selected source line and the unselectedsource line, respectively.

In addition, the illustration has been made of the case where the memorycell array is the cross-point type memory cell array having the 1D1Rstructure in which the diode is provided in the memory cell, or thecross-point type memory cell array having the 1T1R structure in whichthe transistor is provided in the memory cell, in the above embodiment,but the present invention is not limited to this configuration, and aslong as the variable resistive element of the present invention havingthe metal oxide as the resistance change layer and further having thecontrol layer is employed in the memory cell, the present invention canbe applied to any memory cell array in which the above memory cells arearranged in a shape of matrix.

Furthermore, the illustration has been made of the case where theresistance change layer 13 is directly in contact with the secondelectrode 12 as the configuration of the variable resistive element 2,in the above embodiment, but the present invention is not limited tothis. In order to provide a function as a nonlinear current limitelement, a configuration may be provided such that a tunnel insulationfilm is inserted between the second electrode 12 and the resistancechange layer 13, and in order to reduce an element variation of thefilament path formed by the forming process, a configuration may beprovided such that a buffer layer is inserted to prevent the currentflowing between the electrodes of the variable resistive element fromabruptly increasing after the completion of the forming process.

In addition, the illustration has been made of the element structureshown in FIG. 1 as the configuration of the variable resistive element 2in the above embodiment, but the present invention is not limited to theelement having the above structure.

Furthermore, the illustration has been made of the case where theresistance change layer (first metal oxide film) 13 and the controllayer 14 include the oxide films of the different metals or filmscontaining oxygen as the configuration of the variable resistive element2 in the above embodiment, but they may be the oxide films of the samekind of metal or the films containing oxygen. In this case, in a methodfor producing the variable resistive element, the first metal oxide filmserving as the oxide of the first metal is formed as the resistancechange layer, the film of the first metal material is deposited on thefirst metal oxide film, oxygen in the first metal oxide film is moved tothe side of the first metal material by application of heat and aforming voltage, whereby the control layer serving as the first metaloxide is formed. Therefore, each of the resistance change layer and thecontrol layer is the first metal film containing oxygen, but its oxygenconcentration is different from each other. The oxygen concentrationdistribution is provided such that while the concentration reduces fromthe resistance change layer on the side of the second electrode towardthe control layer on the side of the first electrode across the boundarybetween the resistance change layer and the control layer, it shows akink-shaped concentration distribution in which the concentrationabruptly changes in the boundary between the resistance change layer andthe control layer. In other words, a reduction rate of the oxygenconcentration shows a maximum value in the vicinity of the boundarybetween the resistance change layer and the control layer.

The present invention can be applied to a nonvolatile semiconductormemory device, and particularly to a nonvolatile semiconductor memorydevice provided with a nonvolatile variable resistive element in which aresistance state is changed by voltage application, and the resistancestate after changed is kept in a nonvolatile manner.

Although the present invention has been described in terms of thepreferred embodiment, it will be appreciated that various modificationsand alternations might be made by those skilled in the art withoutdeparting from the spirit and scope of the invention. The inventionshould therefore be measured in terms of the claims which follow.

What is claimed is:
 1. A variable resistive element comprising a firstmetal oxide film which is an oxide film of a first metal and sandwichedbetween a first electrode and a second electrode, wherein through aforming process, a resistance state between the first and secondelectrodes of the variable resistive element is changed from an initialhigh resistance state before the forming process to a variableresistance state, the resistance state in the variable resistance stateis changed between two or more different resistance states by applyingan electric stress between the first electrode and the second electrodeof the variable resistive element in the variable resistance state, andone resistance state after the change is used for storing information, acontrol layer containing oxygen is inserted between the first electrodeand the first metal oxide film, and formed of a second metal capable ofextracting oxygen from the first metal oxide film to prevent oxygen frombeing thermally diffused from the first metal oxide film to the firstelectrode, the second metal included in the control layer is differentfrom the first metal, or same as the first metal, when the second metalis same as the first metal, a concentration distribution is provided insuch a manner that an oxygen concentration of the control layer and thefirst metal oxide film becomes lower from the first metal oxide filmtoward the control layer across a boundary between the first metal oxidefilm and the control layer, oxide formation free energy of at least oneelement included in the control layer except for oxygen is lower thanoxide formation free energy of an element included in the firstelectrode, and both work functions of the second metal and the firstelectrode are 4.5 eV or less.
 2. The variable resistive elementaccording to claim 1, wherein a concentration distribution is providedin such a manner that an oxygen concentration of the control layerbecomes lower from a side of the first metal oxide film toward a side ofthe first electrode.
 3. The variable resistive element according toclaim 1, wherein a concentration distribution is provided in such amanner that an oxygen concentration of the first metal oxide filmbecomes lower from a side of the second electrode toward a side of thecontrol layer.
 4. The variable resistive element according to claim 1,wherein the first metal oxide film comprises an n-type metal oxide. 5.The variable resistive element according to claim 4, wherein the firstmetal oxide film comprises an oxide of any one of elements of Hf, Zr,Ti, Ta, V, Nb, and W, or a strontium titanate.
 6. The variable resistiveelement according to claim 1, wherein the second metal comprises any oneof elements of Ti, V, Al, Hf, and Zr.
 7. The variable resistive elementaccording to claim 1, wherein a work function of the second electrode is4.5 eV or more.
 8. The variable resistive element according to claim 7,wherein the second electrode comprises a Ti nitride.
 9. The variableresistive element according to claim 1, wherein the control layer isthinner than the first electrode.
 10. The variable resistive elementaccording to claim 9, wherein a film thickness of the control layer is20 nm or less.
 11. A method for producing the variable resistive elementaccording to claim 1, comprising: depositing a second electrode materialon a substrate, and forming the second electrode; depositing a firstmetal oxide film material, a second metal material, and a firstelectrode material in this order; forming the first metal oxide film andthe first electrode by patterning the first metal oxide film material,the second metal material, and the first electrode material with acommon resist mask; performing a heat treatment; and applying a formingvoltage between the first electrode and the second electrode to performa forming process, whereby oxygen in the first metal oxide film beingpartially moved toward the second metal material, the second metalmaterial being changed into the control layer, a resistance state of thevariable resistive element being changed from the initial highresistance state to the variable resistance state.
 12. A nonvolatilesemiconductor memory device comprising a memory cell array having thevariable resistive elements according to claim 1, the variable resistiveelements being arranged in at least a column direction of a rowdirection and the column direction.
 13. The nonvolatile semiconductormemory device according to claim 12, wherein the memory cell array isprovided in such a manner that the first electrode extends in the columndirection to connect the variable resistive elements adjacent in thecolumn direction, and the control layer extends in the column direction.